Chapter 20 SHEET METALWORKING

1. Chapter 20SHEET METALWORKING • Cutting Operations • Bending Operations • Drawing • Other Sheet Metal Forming Operations • Dies and Presses for Sheet Metal Processes • Sheet Metal Operations Not Performed on Presses • Bending of Tube Stock ISE 316 - Manufacturing Processes Engineering

2. Sheet Metalworking Defined Cutting and forming operations performed on relatively thin sheets of metal • Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in) • Thickness of plate stock > 6 mm • Operations usually performed as cold working ISE 316 - Manufacturing Processes Engineering

3. Sheet and Plate Metal Products • Sheet and plate metal parts for consumer and industrial products such as • Automobiles and trucks • Airplanes • Railway cars and locomotives • Farm and construction equipment • Small and large appliances • Office furniture • Computers and office equipment ISE 316 - Manufacturing Processes Engineering

4. Advantages of Sheet Metal Parts • High strength • Good dimensional accuracy • Good surface finish • Relatively low cost • For large quantities, economical mass production operations are available ISE 316 - Manufacturing Processes Engineering

5. Sheet Metalworking Terminology • “Punch‑and‑die” • Tooling to perform cutting, bending, and drawing • “Stamping press” • Machine tool that performs most sheet metal operations • “Stampings” • Sheet metal products ISE 316 - Manufacturing Processes Engineering

6. Three Major Categories of Sheet Metal Processes • Cutting • Shearing to separate large sheets; or cut part perimeters or make holes in sheets • Bending • Straining sheet around a straight axis • Drawing • Forming of sheet into convex or concave shapes ISE 316 - Manufacturing Processes Engineering

7. Cutting Shearing between two sharp cutting edges Figure 20.1 ‑ Shearing of sheet metal between two cutting edges: (1) just before the punch contacts work ISE 316 - Manufacturing Processes Engineering

8. Figure 20.1 ‑ Shearing of sheet metal between two cutting edges: (2) punch begins to push into work, causing plastic deformation ISE 316 - Manufacturing Processes Engineering

9. Figure 20.1 ‑ Shearing of sheet metal between two cutting edges: (3) punch compresses and penetrates into work causing a smooth cut surface ISE 316 - Manufacturing Processes Engineering

10. Figure 20.1 ‑ Shearing of sheet metal between two cutting edges: (4) fracture is initiated at the opposing cutting edges which separates the sheet ISE 316 - Manufacturing Processes Engineering

11. Shearing, Blanking, and Punching Three principal operations in pressworking that cut sheet metal: • Shearing • Blanking • Punching ISE 316 - Manufacturing Processes Engineering

12. Shearing Sheet metal cutting operation along a straight line between two cutting edges • Typically used to cut large sheets into smaller sections for subsequent operations ISE 316 - Manufacturing Processes Engineering

13. Figure 20.3 ‑ Shearing operation: • side view of the shearing operation (b) front view of power shears equipped with inclined upper cutting blade Symbol v indicates motion ISE 316 - Manufacturing Processes Engineering

14. Blanking and Punching Blanking - sheet metal cutting to separate piece from surrounding stock • Cut piece is the desired part, called a blank Punching - sheet metal cutting similar to blanking except cut piece is scrap, called a slug • Remaining stock is the desired part ISE 316 - Manufacturing Processes Engineering

15. Figure 20.4 ‑ (a) Blanking and (b) punching ISE 316 - Manufacturing Processes Engineering

16. Clearance in Sheet Metal Cutting Distance between the punch and die • Typical values range between 4% and 8% of stock thickness • If too small, fracture lines pass each other, causing double burnishing and larger force • If too large, metal is pinched between cutting edges and excessive burr results ISE 316 - Manufacturing Processes Engineering

17. Clearance in Sheet Metal Cutting • Recommended clearance can be calculated by: c = at where c = clearance; a = allowance; and t = stock thickness • Allowance a is determined according to type of metal ISE 316 - Manufacturing Processes Engineering

18. Allowance a for Three Sheet Metal Groups ISE 316 - Manufacturing Processes Engineering

19. Punch and Die Sizes for Blanking and Punching For a round blank of diameter Db: Blanking punch diameter = Db ‑ 2c Blanking die diameter = Db where c = clearance For a round hole of diameter Dh: Hole punch diameter = Dh Hole die diameter = Dh + 2c where c = clearance ISE 316 - Manufacturing Processes Engineering

20. Figure 20.6 ‑ Die size determines blank size Db; punch size determines hole size Dh.; c = clearance ISE 316 - Manufacturing Processes Engineering

21. Angular Clearance Purpose: allows slug or blank to drop through die • Typical values: 0.25 to 1.5 on each side Figure 20.7 ‑ Angular clearance ISE 316 - Manufacturing Processes Engineering

22. Cutting Forces Important for determining press size (tonnage) F = S t L where S = shear strength of metal; t = stock thickness, and L = length of cut edge ISE 316 - Manufacturing Processes Engineering

23. Bending Straining sheetmetal around a straight axis to take a permanent bend Figure 20.11 ‑ (a) Bending of sheet metal ISE 316 - Manufacturing Processes Engineering

24. Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched Figure 20.11 ‑ (b) both compression and tensile elongation of the metal occur in bending ISE 316 - Manufacturing Processes Engineering

25. Types of Sheetmetal Bending • V‑bending - performed with a V‑shaped die • Edge bending - performed with a wiping die ISE 316 - Manufacturing Processes Engineering

26. V-Bending • For low production • Performed on a press brake • V-dies are simple and inexpensive Figure 20.12 ‑ (a) V‑bending ISE 316 - Manufacturing Processes Engineering

27. Edge Bending • For high production • Pressure pad required • Dies are more complicated and costly Figure 20.12 ‑ (b) edge bending ISE 316 - Manufacturing Processes Engineering

28. Stretching during Bending • If bend radius is small relative to stock thickness, metal tends to stretch during bending • Important to estimate amount of stretching, so that final part length = specified dimension • Problem: to determine the length of neutral axis of the part before bending ISE 316 - Manufacturing Processes Engineering

29. Bend Allowance Formula where BA = bend allowance; A = bend angle; R= bend radius; t = stock thickness; and Kba is factor to estimate stretching • If R < 2t, Kba = 0.33 • If R  2t, Kba = 0.50 ISE 316 - Manufacturing Processes Engineering

30. Springback in Bending Springback = increase in included angle of bent part relative to included angle of forming tool after tool is removed • Reason for springback: • When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape ISE 316 - Manufacturing Processes Engineering

31. Figure 20.13 ‑ Springback in bending shows itself as a decrease in bend angle and an increase in bend radius: (1) during bending, the work is forced to take the radius Rb and included angle Ab' of the bending tool (punch in V‑bending), (2) after punch is removed, the work springs back to radius R and angle A' ISE 316 - Manufacturing Processes Engineering

32. Bending Force Maximum bending force estimated as follows: where F = bending force; TS = tensile strength of sheet metal; w = part width in direction of bend axis; and t = stock thickness. For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33 ISE 316 - Manufacturing Processes Engineering

33. Figure 20.14 ‑ Die opening dimension D: (a) V‑die, (b) wiping die ISE 316 - Manufacturing Processes Engineering

34. Drawing Sheet metal forming to make cup‑shaped, box‑shaped, or other complex‑curved, hollow‑shaped parts • Sheet metal blank is positioned over die cavity and then punch pushes metal into opening • Products: beverage cans, ammunition shells, automobile body panels ISE 316 - Manufacturing Processes Engineering

35. Figure 20.19 ‑ • Drawing of a cup‑shaped part: • start of operation before punch contacts work • near end of stroke (b) Corresponding workpart: (1) starting blank (2) drawn part ISE 316 - Manufacturing Processes Engineering

36. Clearance in Drawing • Sides of punch and die separated by a clearance c given by: c = 1.1 t where t = stock thickness • In other words, clearance = about 10% greater than stock thickness ISE 316 - Manufacturing Processes Engineering

37. Drawing Ratio DR where Db = blank diameter; and Dp = punch diameter • Indicates severity of a given drawing operation • Upper limit = 2.0 Most easily defined for cylindrical shape: ISE 316 - Manufacturing Processes Engineering

38. Reduction r • Again, defined for cylindrical shape: • Value of r should be less than 0.50 ISE 316 - Manufacturing Processes Engineering

39. Thickness‑to‑Diameter Ratio Thickness of starting blank divided by blank diameter Thickness-to-diameter ratio = t/Db • Desirable for t/Db ratio to be greater than 1% • As t/Db decreases, tendency for wrinkling increases ISE 316 - Manufacturing Processes Engineering

40. Blank Size Determination • For final dimensions of drawn shape to be correct, starting blank diameter Db must be right • Solve for Db by setting starting sheet metal blank volume = final product volume • To facilitate calculation, assume negligible thinning of part wall ISE 316 - Manufacturing Processes Engineering

41. Shapes other than Cylindrical Cups • Square or rectangular boxes (as in sinks), • Stepped cups, • Cones, • Cups with spherical rather than flat bases, • Irregular curved forms (as in automobile body panels) • Each of these shapes presents its own unique technical problems in drawing ISE 316 - Manufacturing Processes Engineering

42. Other Sheet Metal Forming on Presses Other sheet metal forming operations performed on conventional presses • Operations performed with metal tooling • Operations performed with flexible rubber tooling ISE 316 - Manufacturing Processes Engineering

43. Ironing • Makes wall thickness of cylindrical cup more uniform • Examples: beverage cans and artillery shells Figure 20.25 ‑ Ironing to achieve a more uniform wall thickness in a drawn cup: (1) start of process; (2) during process Note thinning and elongation of walls ISE 316 - Manufacturing Processes Engineering

44. Embossing • Used to create indentations in sheet, such as raised (or indented) lettering or strengthening ribs Figure 20.26 ‑ Embossing: (a) cross‑section of punch and die configuration during pressing; (b) finished part with embossed ribs ISE 316 - Manufacturing Processes Engineering

45. Guerin Process Figure 20.28 ‑ Guerin process: (1) before and (2) after Symbols v and F indicate motion and applied force respectively ISE 316 - Manufacturing Processes Engineering

46. Advantages of Guerin Process • Low tooling cost • Form block can be made of wood, plastic, or other materials that are easy to shape • Rubber pad can be used with different form blocks • Process attractive in small quantity production ISE 316 - Manufacturing Processes Engineering

47. Dies for Sheet Metal Processes Most pressworking operations performed with conventional punch‑and‑die tooling • Custom‑designed for particular part • The term stamping die sometimes used for high production dies ISE 316 - Manufacturing Processes Engineering

48. Figure 20.30 ‑ Components of a punch and die for a blanking operation ISE 316 - Manufacturing Processes Engineering

49. Figure 20.31 ‑ • Progressive die; • associated strip development ISE 316 - Manufacturing Processes Engineering

50. Figure 20.32 ‑ Components of a typical mechanical drive stamping press ISE 316 - Manufacturing Processes Engineering